Memory processing in humans, monkeys, and pigeons will be studied in a serial probe recognition task employing a 2000 item pool. Serial position effects will be explored through: controlled, interference, item presentation rate, bias and list differentiation manipulations, delayed recognition, variable list lengths, and item repetitions. Crucial stimulus features of the color photograph items will be explored through: cluster analysis and multidimensional scaling, eye movement analysis, and controlled image degradation.